After enjoying this wonderful video of a Vigier fretless guitar with a metal neck I have decided to defret one of my cheaper Les Paul copies to see what it feels like, but I have a couple of queries, as I have never defretted a neck before.

Roundwound bass strings may be thick, but they still chew up the wood after a while, especially if it's something softer than ebony. I believe the treble strings, much like flatwounds, are less of a problem in that account.
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leftaroundaboutJun 11 '12 at 15:32

Regarding the strings, you might go up a guage or three. And you might want to lower the action - especially at the nut. If you guage up from 9s with a factory nut you might want to slightly widen the slots anyway so a set of nut files from Stew-Mac would let you drop that down slightly at the same time. My gut says I might want 12s on a fretless just to have more mass and sustain, but I've never played one.
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Todd WilcoxApr 8 at 12:55

2 Answers
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One thing not mentioned in the post you link to about de-fretting a bass guitar is that after the frets are removed, and the fret slots are filled, it is common to apply a thick hard finish to the rosewood fretboard with marine epoxy or super glue (cyanoacrylate). This usually involves multiple coats of the finish and drying and fine sanding inbetween layers, followed by buffing to create a glossy surface. The purpose of the finish, I have read, is to protect the rosewood from being "chewed up" by the roundwound strings, but also to increase sustain and brighten the tone of the instrument.

Marine epoxy coating on the de-fretted fingerboard was popularized by pioneering fretless jazz bassist Jaco Pastorius. In more recent years cyanoacrylate has become a more common alternative.

You can find discussions on finishing a fingerboard in epoxy or cyanoacrylate on talkbass.com.

I converted an acoustic, and even replaced nylon strings with steel to add pick-ups, and yet this (fretboard wear) doesn't appear to be an issue at all. There are two reasons for this being so:

First, I filed off my frets instead of removing them, so there is still metal beneath the areas with most stress.

Second, becoming accurate with fretless playing is more of an analog slide process than a digital fret business. You'll become proficient in sliding perpendicular to the strings to achieve your desired note, and adopt that technique or trills (rocking your finger) in place of using bends which are the main culprit in fretboard wear.

Your sound will suffer however if you continue to play with the pads of your fingers. You will have quieter notes with less attack, less sustain, and less succinct tonality. To resemble the tone you had with frets will require playing with your nails pressed against the fretboard, which is no easy trick.

Also your right-hand technique will have to change even if you increase the action. You had some action space between frets before with frets. Now your strings will rest directly on the fretboard, and any plucking or strumming short of perpendicular to the surface is going to result in string slap or buzz.

Wood is softer, and metal would be nicer, but you'll be varnishing the thing anyway. I suppose the stiffer the varnish, the better. I used marine spar varnish and it worked fine. The fine powder from filing my nickel frets got into the wood grain, but I find I prefer the look.

My issue was that I started with a guitar which had already been gouged out from bending. It wouldn't have been an issue except that it was local to three strings and three frets, and i have to adjust my intervals at that one scooped out spot. Your fretboard may never have been level to begin with (mfgr's only need to level frets, not fretboards), but that much can be remedied with more varnish and a long metal sanding block, and one can adapt playing to 'gradual' inconsistencies. All told, I'm so glad I finally have a versatile instrument which suits my slide style and foreign scales.
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Kristal McKinstryApr 8 at 11:23